The present invention relates to a motor and especially to a technique for maintaining constant power consumption in preheating a compressor motor in an air conditioner. It also relates to a technique for preheating a compressor motor driven by a pulse-width modulation inverter.
Conventionally, lubricating oil has been employed for reducing friction at a motor bearing. This technique is also employed in compressor motors in air conditioners.
The compressor motors in air conditioners in particular handle refrigerant and have the characteristic that at low temperatures, the refrigerant is soluble in refrigeration oil which is lubricating oil. When driven at low temperatures, the compressor motors in air conditioners rotate with the refrigeration oil in small concentrations. Thus, there is a high possibility that sliding portions of the compressors will seize up by friction.
To prevent such a problem, there has been employed a technique for providing a crank heater around a compressor motor to preheat the motor before it rotates and thereby to reduce the solubility of refrigeration oil in refrigerant. Further, especially for the purpose of preheating a compressor motor driven by an inverter, there has also been employed a technique for, without providing a crank heater, supplying current from the inverter to the compressor motor on the condition that the motor should not be rotated. The current waveform not to rotate the motor is achieved, for example, by increasing frequency while reducing supply current or by employing direct current.
The technique in which the inverter supplies current on the above condition so that the compressor motor preheats itself, however, has a problem of being highly susceptible to variations in supply power. For example if the current waveform is determined to give 35 W of power consumption by preheating for a receiving voltage of 200 V, an increase of the receiving voltage to 220 V increases the power consumption by preheating to approximately 42 W, i.e., by a factor of (220/200)2. This results in excessive power consumption. A decrease of the receiving voltage to 180 V, on the other hand, reduces the power consumption by preheating to approximately 28 W, i.e., by a factor of (180/200)2. This results in insufficient preheating and thereby raises the possibility that the solubility of refrigeration oil in refrigerant cannot be reduced sufficiently.
The present invention has been made in view of the above circumstances and provides a technique for permitting constant preheating of a motor irrespective of variations in receiving voltage. The present invention also provides a technique for giving constant preheating power irrespective of a temperature of a coil of a motor.
A first aspect of the present invention is directed to a method of controlling preheating power, the method controlling preheating of a coil (LU, LW) of a polyphase motor (30), wherein the polyphase motor is operated open-phase to generate heat.
According to a second aspect of the present invention, in the method of controlling preheating power according to the first aspect, the polyphase motor is driven by an inverter (15), the inverter applying current to the coil (LU, LW) for the preheating of the polyphase motor, the current being applied with a predetermined period (T) from the inverter to the polyphase motor, on a condition that the polyphase motor should not be rotated, and as a DC voltage (Vm) applied to the inverter increases, a duty (D) which is the ratio between time during which the voltage is applied to the coil and the period, is set to a smaller value.
According to a third aspect of the present invention, in the method of controlling preheating power according to the second aspect, the current produces no revolving field for the polyphase motor in preheating the polyphase motor.
According to a fourth aspect of the present invention, in the method of controlling preheating power according to either one of the first through third aspects, the polyphase motor is a compressor motor in an air conditioner.
According to a fifth aspect of the present invention, in the method of controlling preheating power according to the second aspect, the polyphase motor is a three-phase motor, the inverter is a three-phase inverter, each phase having a pair of positive and negative switching elements (QU, QV, QW, QX, QY, QZ), and the period includes: a first period (ton) during which the positive switching element (QU) in a first phase (U) is in the on state, the negative switching element (QZ) in a second phase (W) is in the on state, and the positive and negative switching elements (QV; QY) in a third phase (V) are complementarily in the on state for an equal length of time; and a second period (toff) during which, in all of the first, second, and third phases, either the positive switching elements (QU, QV, QW) are in the on state or the negative switching elements (QX, QY, QZ) are in the on state.
According to a sixth aspect of the present invention, in the method of controlling preheating power according to either one of the second and fifth aspects, the duty is set to a value obtained by dividing a product of a known calibrated voltage (Vref), a known calibrated current (Iref), and a known calibrated duty (D0) by a product of a current (Im) and a voltage (Vm) applied to the motor.
According to a seventh aspect of the present invention, in the method of controlling preheating power according to the second aspect, the duty is set to a larger value as a temperature of the polyphase motor increases.
According to an eighth aspect of the present invention, in the method of controlling preheating power according to the second aspect, the preheating is performed on refrigeration oil in a compressor.
A ninth aspect of the present invention is directed to a preheating system comprising: a polyphase motor (30) having coils (LU, LW); and an operation control unit for operating the polyphase motor open-phase to generate heat.
According to a tenth aspect of the present invention, in the preheating system according to the ninth aspect, the operation control unit includes an inverter (15), the inverter applying current to the polyphase motor with a predetermined period (T), on a condition that the polyphase motor should not be rotated, and as a DC voltage (Vm) applied to the inverter increases, a duty (D) which is the ratio between time during which the voltage is applied to the coil and the period, is set to a smaller value.
According to an eleventh aspect of the present invention, in the preheating system according to the tenth aspect, in the preheating of the polyphase motor, the current produces no revolving field for the polyphase motor.
According to a twelfth aspect of the present invention, in the preheating system according to either one of the ninth through eleventh aspects, the polyphase motor is a compressor motor in an air conditioner.
According to a thirteenth aspect of the present invention, in the preheating system according to the tenth aspect, the polyphase motor is a three-phase motor, the inverter is a three-phase inverter, each phase having a pair of positive and negative switching elements (QU, QV, QW, QX, QY, QZ), and the period includes: a first period (ton) during which the positive switching element (QU) in a first phase (U) is in the on state, the negative switching element (QZ) in a second phase (W) is in the on state, and the positive and negative switching elements (QV; QY) in a third phase (V) are complementarily in the on state for an equal length of time; and a second period (toff) during which, in all of the first, second, and third phases, either the positive switching elements (QU, QV, QW) are in the on state or the negative switching elements (QX, QY, QZ) are in the on state.
According to a fourteenth aspect of the present invention, in the preheating system according to either one of the tenth and thirteenth aspects, the duty is set to a value obtained by dividing a product of a known calibrated voltage (Vref), a known calibrated current (Iref), and a known duty (D0) by a product of a current (Im) and a voltage (Vm) applied to the motor.
According to a fifteenth aspect of the present invention, in the preheating system according to the tenth aspect, the duty is set to a larger value as a temperature of the polyphase motor increases.
According to a sixteenth aspect of the present invention, in the preheating system according to the tenth aspect, the preheating is performed on refrigeration oil in a compressor.
In the method of controlling preheating power according to the first aspect of the present invention and in the preheating system according to the ninth aspect, the polyphase motor is operated open-phase to generate heat and thereby to be preheated.
In the method of controlling preheating power according to the second aspect of the present invention and in the preheating system according to the tenth aspect, since variations in the DC voltage applied to the inverter are compensated by varying the duty, preheating can be achieved with predetermined power irrespective of the variations in the DC voltage. This avoids excess power consumption and insufficient preheating.
In the method of controlling preheating power according to the third aspect of the present invention and in the preheating system according to the eleventh aspect, since no revolving field is provided for the polyphase motor, the polyphase motor can be preheated without being rotated. This prevents the occurrence of such mutually contradictory phenomena that heat generation causes the bearing to be worn out. Besides, the generation of sound can be minimized because a so-called xe2x80x9cslipxe2x80x9d is not utilized which is a phenomenon that the revolving field is produced but the motor is not rotated.
In the method of controlling preheating power according to the fourth aspect of the present invention and in the preheating system according to the twelfth aspect, the solubility of lubricating oil in refrigerant which is handled by the compression motor in the air conditioner, can be reduced. This prevents a situation in which, with refrigeration oil which is lubricating oil being dissolved in refrigerant and reduced in concentration, the motor is rotated and thereby the bearing will seize up by friction.
In the method of controlling preheating power according to the fifth aspect of the present invention and in the preheating system according to the thirteenth aspect, during the first period, the positive and negative switching elements in the third phase are complementarily in the on state for an equal length of time; therefore, substantially no current flows to the coil in the third phase in the polyphase motor. During the second period, the inductance of the coils in the first and second phases in the motor maintains the current flow occurring during the first period. Accordingly, current continues to flow in the same direction to the series connection of the coils in the first and second phases, which allows almost constant current to continuously flow to the motor without generating a revolving field.
In the method of controlling preheating power according to the sixth aspect of the present invention and in the preheating system according to the fourteenth aspect, the duty is the value obtained by dividing desired power by the product of current and voltage applied to the motor. This allows constant preheating power to be given irrespective of the values of current and voltage applied to the motor.
With increase in temperature, the DC resistance of the coil increases and the current flowing to the motor decreases. In the method of controlling preheating power according to the seventh aspect of the present invention and in the preheating system according to the fifteenth aspect, however, preheating power can be increased by increasing the duty. Accordingly, the preheating power can be set to a desired value irrespective of the temperature of the polyphase motor.
In the method of controlling preheating power according to the eighth aspect of the present invention and in the preheating system according to the sixteenth aspect, refrigeration oil is hardly soluble in refrigerant and thus the motor can be prevented from being rotated with the refrigeration oil reduced in concentration. This prevents a sliding portion of the compressor from seizing up by friction.